1. Field of the Invention
The present invention relates to the development and manufacture of masks for semiconductor fabrication and more specifically to a test structure, method and apparatus for measuring the effective phase of attenuating phase shift masks.
2. Background of the Invention
As metrology tolerances for photomasks become increasingly tight, the issues of measurement methods and calibration standards become more important. This is as true for optical phase metrology as it is for dimensional metrology.
An optical phase standard can be defined using an alternating phase shift mask as shown in FIG. 1. FIG. 1 shows an example of an alternating phase shift mask 100 used for an optical phase standard. Mask 100 is created by etching a step or a trench 120 into a transparent medium substrate 110, for example, fused silica, with a well-characterized index of refraction (as shown in FIG. 2a). The normal-incidence phase shift of such a structure can be calculated from the trench depth and the index of refraction. The measurement accuracy is mainly limited by the ability to measure the physical depth of the trench. The aerial image of such a structure is shown in FIG. 2b. 
Other methods exist for measuring optical phase shifts in alternating aperture masks. For example a Levenson grating exhibits symmetrical behavior through focus only if the phase difference between the phase shifted and unshifted openings is exactly 180°.
A measured asymmetry, through focus, between the phase shifted and unshifted clear features in such a grating can be used to quantitatively measure the deviation from 180° optical phase. This approach can use measurements taken on an aerial image metrology system with the numerical aperture and illumination conditions used by the wafer stepper that will eventually project the mask, or actual exposures of wafers on a stepper. However, calibration of optical phase in attenuating phase masks is more difficult and current alternating phase shift calibration methods are inadequate and inaccurate for measuring phase shift in masks with very small structures.